The complete combustion of a hydrocarbon produces carbon dioxide and water.

The idea

Complete combustion happens when a hydrocarbon burns in a plentiful supply of oxygen. The carbon atoms in the fuel are oxidised to carbon dioxide, and the hydrogen atoms are oxidised to water. For example, burning methane in excess oxygen gives carbon dioxide and water. The reaction releases energy, which is why hydrocarbons are useful as fuels. If oxygen is limited, combustion is incomplete and carbon monoxide or soot may form instead, so the products named here apply specifically to complete combustion. Testing the products confirms the idea: bubbling the gases through limewater turns it milky, showing carbon dioxide, while water vapour condenses as colourless droplets that turn anhydrous copper(II) sulfate from white to blue.

What earns the marks

• Complete combustion requires a plentiful supply of oxygen.
• The carbon in the hydrocarbon is oxidised to carbon dioxide.
• The hydrogen in the hydrocarbon is oxidised to water.
• The reaction is exothermic, releasing energy.
• Carbon dioxide can be identified because it turns limewater milky.
• Water can be identified because it turns anhydrous copper(II) sulfate from white to blue.

Common mistakes and how to put them right

• Writing that complete combustion produces carbon monoxide or soot.

  Put it right by stating that those products indicate incomplete combustion with limited oxygen.

• Naming water as hydrogen oxide or hydrogen gas.

  Put it right by naming the product water, formula H₂O.

• Forgetting that oxygen is a reactant.

  Put it right by including oxygen on the left-hand side of the word equation.

Exam tips

• State clearly that the oxygen supply is plentiful when describing complete combustion.
• Give both products, carbon dioxide and water, in the correct order from the word equation.
• If asked to test a product, name the reagent and the observed colour change rather than just the gas.